A "modem pool" is a group of modems which are normally used to
receive incoming calls. Today, many such modems may be on a single
card. ISPs once used modem pools so that customers could call in to
the ISP, but today, the RAS (Remote Access Server) has replaced modem
pools for ISPs. RAS works for incoming calls at near 56k (V.90 and
V.92) and uses what amounts to "digital modems". Modem pools use the
older analog modems and can only go to 33.6 kbps for incoming calls.
Thus analog modem pools are more likely to be used by small
organizations that don't want to use the more expensive RAS. A RAS is
in a sense a digital modem pool.

An analog modem pool may be provided by an analog multi-port modem card.
In olden days it was many modems in an external chassis (something
like an external modem). Such modems could be analog modems similar to
modems used for home/office PCs (can't send above 33.6k even if they
are labeled "56k modems"). A RAS will use "digital modems" which
can send at nearly 56k (if you have a good line). The "digital
modems" require a digital connection to the telephone line and don't
use any serial ports at all. All of these modem pools (or RAS's) will
require that you install special drivers for them.

A "multimodem card" is short for "multiport modem card". Some put
a hyphen after "multi": multi-modem or multi-port. An analog modem
pool is just many analog modems (the common home/office modem)
provided either on an internal plug-in card or in an external chassis.
Each modem comes with a built-in serial port. There is usually a
system of sharing interrupts or of handling interrupts by their own
electronics, thus removing much of this burden from the CPU. Note
that these modems are not "digital modems" and will thus not be able
to use 56k for people who dial-in.

Here is a list of some companies that make analog multiport modem
cards which plug into slots in a PC. 8 modems/card is common. The
cards listed claim to work with Linux and the websites should point
you to a driver for them.

"Digital modems" are much different than the analog modems that
most people use in their PCs. But they can communicate with analog
modems at the other end of the phone line. ISP's use "digital modems"
to send out data at almost 56k bps to 56k modems in homes and offices.
The "digital modem" requires a digital connection to the telephone
line (such as T1, EI, ISDN PRI, etc.). Except for some serial ISDN
"modems", they don't use serial ports for the interface to the
computer. Instead, they interface directly to a computer bus via a
special card(s) (which may also contain the "digital modems"). They
are often a component of "remote access servers" (RASs) or "digital
modem pools"

You may already know that each time you make a telephone call from an
analog device (a telephone or a modem) it gets converted by the
telephone company to a digital signal. Then it's transmitted to near
its destination as a digital signal and finally converted back into an
analog signal just before it reaches it's destination. But it's also
possible to receive this digital signal directly from the telephone
company if you have what is called a "T1" line, etc.

The cables from the phone company that carry digital signals have been
designed for high bandwidth so that the same cable carries many
telephone calls. It's done by what's called "time-division
multiplexing". A single phone call in a cable is carried on two
different channels, one for each direction. So the RAS must connect
each such channel-pair to the appropriate "digital modem" that
services that phone call. Such tasks are done by what is sometimes
called a "... concentrator".

Now the digital signal received by a "digital modem" may really
represent an analog signal which has been sent to it by an analog
modem. This is because when you send an analog signal (including
ordinary voice) to the telephone company, it gets converted into
digital by the phone company. One way for the digital modem to deal
with this digital signal would be to convert it to an analog signal
and then put that thru an analog modem to get the digital data sent by
the analog modem. But why do all this work? Since the signal is
already in digital form, why not process it digitally? That's how
it's done. The digital signal is processed and converted to another
digital stream of bytes which represents data bytes sent by the analog
modem. A "digital signal processor" (DSP) is commonly used for this
task. A CPU could also handle it but it would be heavily loaded.

Likewise, a "digital modem" must handle sending digital signals in the
opposite direction from a RAS to a digital telephone line. Thus it
only makes digital-to-digital conversions and doesn't deal in analog
at all. It thus is not really a modem at all since it doesn't
modulate any analog carrier. So the name "digital modem" is a
misnomer but it does do the job formerly done by modems. Thus some
"digital modems" call themselves "digital signal processors", or
"remote access servers", etc. and may not even mention the word
"modem".

Such a RAS system may be a stand-alone proprietary server, a chassis
containing digital modems that connects to a PC via a special
interface card, or just a card itself. Digi calls one such card a
"remote access server concentrator adapter". One incomplete
description of what is needed to become an ISP is: See
What do I need to be an ISP?. Cyclades promotes their own
products here so please do comparison shopping before buying anything.